Inferred (result) type of overridden member should not be narrower than in super interface

[scabug]

Discussion at https://groups.google.com/d/msg/scala-internals/6vemF4hOA9A/tsouPjHDO_AJ

scala> trait Foo { def foo: Option[String] }
defined trait Foo

scala> trait Bar extends Foo { def foo = Some("bar") }
defined trait Bar

scala> trait Baz extends Bar { override def foo: Option[String] = Some("baz") }
<console>:9: error: overriding method foo in trait Bar of type => Some[String];
 method foo has incompatible type
       trait Baz extends Bar { override def foo: Option[String] = Some("baz") }
                                            ^

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-7873?orig=1
Reporter: @som-snytt
Affected Versions: 2.11.0-M5

[scabug]

@paulp said:
FYI this is #7212.

[scabug]

@adriaanm said:
See #7212